actually assuming that u are using bulkheads as long as the water line cannot go any higher then 30" then 1/2" will be fine. are u sure your 115 aint outta 10mm(3/8) intead of 12mm(1/2")
If you are limited by the glass you can afford then consider moving the window higher up the tank side. The higher up the window, the less water pressure there is. The wooden/fiberglass tank can be as deep as you like below the window but this may make fish viewing akward.
Unfortunately the water pressure isnt the only consideration as some large fish will thump glass pretty hard too.
